This invention relates to a sensors for coil wound devices, and in particular to a sensor for monitoring the status of the insulation on the wire in a coil winding.
There are a number of systems where the continued operation of a coil wound device, such as an electric motor or transformer is critical. For example, in an assembly line, an unanticipated failure of the electric motor results in the shut-down of the entire system. In these applications it would be desirable to have a warning of the imminent failure of the coil so that it could be replaced during a scheduled maintenance period for the device, rather than during operating time. One of the principle causes of coil failure is the failure of the insulation on the wire in windings. Over time, heat and vibration cause the electric insulation on the winding wire to break down, resulting in a short-circuiting of the winding.
Various attempts have been made to check or test the status of the insulation on windings. Most of these efforts focus on periodically applying a test voltage to the winding and measuring for leakage indicative of failed insulation. See, e.g., Horvath et al., U.S. Pat. No. 4,540,922, and Zelm, U.S. Pat. Nos. 5,155,441 and 5,172,289. However these methods usually require that the motor be taken out of service. Other efforts involved measuring the field properties of the winding, e.g. Palueff, U.S. Pat. No. 1,992,013 and Miller et al., U.S. Pat. No. 5,032,826, or measuring the motor operating variables, e.g., Kueck et al., U.S. Pat. No. 5,612,601. Still other efforts involved predicting failure by tracking the operating time and temperature of the motor, e.g., Chu et al, U.S. Pat. No. 5,019,760. Finally, sensing devices can be incorporated into the winding wire itself, e.g. Tokuda et al., U.S. Pat. No. 4,737,775. However, this could add significantly to the cost of the motor, and might affect operation of the motor.
The present invention relates to an apparatus for, and a method of, monitoring the status of the insulation on the wire in a winding, such as the winding of an electric motor, electric generator, or an electric transformer. Generally, the apparatus comprises a surrogate sample of insulating material having a thickness and properties selected to fail before the failure of the insulation on the wire in the winding, and a detector for detecting and indicating failure of the sample of insulating material. In the preferred embodiment, the detector comprises first and second contacts, preferably first and second wires twisted together, and the surrogate sample is preferably a layer of insulation on one or both wires. The detector measures conduction between the wires which indicates failure of the surrogate sample of insulating material.
The surrogate sample is preferably the same material as the insulation on the winding wire, but is thinner than the insulation on the winding wire so that it will fail before the insulation on the winding wire fails. The surrogate sample may be entirely on one of the wire contacts, or it may be on both wire contacts. The surrogate sample is associated with the winding, i.e. it is positioned relative to the winding that it encounters substantially the same environmental conditions as the winding. The surrogate sample is preferably embedded in the winding itself, but it could be positioned external of the winding.
According to the method of this invention, the surrogate sample of insulating material is associated with the winding in a dynamoelectric device such as a motor or generator, or in a transformer, and the sample is monitored. This is conveniently done by incorporating the sample into a probe that also includes first and second contacts. When the surrogate sample of insulation fails, as indicated by conduction between the first and second contacts, it indicates that failure of the winding is likely to occur. The device can then be repaired or replaced before an actual breakdown of the coil occurs.
The apparatus and method of the present invention allow continuous monitoring of the condition of the insulation on the wire in a winding without the need to take the winding out of service for testing. The apparatus and method employ a surrogate sample of insulating material so they do not interfere with the normal construction and assembly of the device in which they are incorporated. However, because the sample is associated with the winding, it provides an accurate representation of the condition of the insulation on the winding wire, rather than a prediction. The apparatus and method measure the insulating property of the surrogate samplexe2x80x94the critical property of the insulation on the winding wire, rather than secondary effects such as changes in the winding field properties which are only evident after the breakdown has begun, and thus the apparatus and method of the present invention provide a more reliable advance notice of winding failure. These and other features and advantages will be in part apparent and in part pointed out hereinafter.